I would suggest to use resistivity surveys. If there is an underground passage in this mound you for sure will detect it. Mound  probably has low resistivity, passage will have high resistivity (air filled).

I suggest you a survey by using in combination the Electrical Resistivity Tomography (ERT) and the Ground Penetrating Radar (GPR) as surface geophysical techniques, in order to investigate the electrical resistivity and the dielectric permittivity of the mound. The frequency range of GPR antenna may be in the range 200-300 MHz.

YOU CAN USE GPR METHODE (GROUND PENETRAITING RADAR) 

You have to carry out all three surveys one after the other. Tomographic Georadar, Geoelectric Tomography and Geoacoustic Tomography. In all cases you need to use the appropriate software tools correctly. We did that both in investigating the Xerxes Canal in East Chalkidiki Peninsula, and the Kastas artificial hill in Amphipolis in Greece.

Actually, you should use multiple techniques only if it can potentially add relevant information, often it is not worth the additionnal cost (especially if you plan to make it done by a private compagny). If we review the available methods we have :

• GPR (radar) : might be your solution, if there is a passage, filled or not, it should most probably act as a reflector BUT be careful of the composition of the soil : clays are hardly penetrated (absorption) and you won't go pass the water level (total reflection)
• Seismic (or geoacoustic) : I would not recommand it in this context. It is a great tool to see horizontal interfaces, such as layers, but is poorly sensible to lateral variations. Moreover, using a vibrating or impact source might not be well accepted by the local authorities/inhabitant
• ERT (electrical tomography) : might also work, be careful though, your result will be one of an inverse problem, if the mound is homegeneous it is not a problem, if it is not (anthropological pollution for example), your solution might be very wrong. The penetration (here 12m) can also be an issue
• Electro-magnetic : I'm not an expert on that, and there are a lot of techiques using the principle. But I would say :urban area = electromagnetic disturbances? better see an expert
• Gravimetry : if the passage is not filled, this is your best option, otherwise it depends on the filling
• Geomagnetics : should work if there is not too much metallic pollution

But this is about theory, another important factor is the logistic/cost : for example geomagnetic mapping can be carried really quickly, you just walk with the device and cover the whole area, GPR is also quite fast, maybe less uneven-ground friendly.

On the other hand for the ERT you need to set up a profile and wait quite a long time before the measurement is done, for gravimetry the waiting occurs for each point.

So, if you really have no idea where the passage might be I would advise to first conduct a large scale survey using either geomagnetics or GPR. If the results is not totally conclusive it will allow you to target some point of interest where you could conduct gravimetry or ERT

Given the parameters I would say GPR.  If you run transects perpendicular to the expected bearing of the passage you should be able to locate it and track it to/from the mound. 

From my scientific and practical experience, I would like to suggest to use the very modern Technique of " New applied and innovative mathematical theory in seismology, geophysics and geotechnical ", developed by me. Where it's initial results depends on measurements of Global Seismological Method ( Microtremor Field Measurements ). Please to inquire about more informations and succesful applications, please e- mail to me by the following e-mail: [email protected]

I suggest you a survey by using Electrical Resistivity Tomography with different electrode spacing. An accurate topographic correction is very important.

I totally agree with Marco about using Electrical Resistivity Tomography. Is fastest , cheapest and most effective geophysical survey (in my opinion) in your case. In Albania we use successfully a method of survey and interpretation called  Resistivity "Real Section" witch is an physical survey of Resistivity values with no inversion. I'm sending to you an Resistivity "Real Section" done to find an underground tunnel. like you can see is very accurate and with good results. All the best!

I will also agree with the fact that ERT might give really good results, but let's stay impartial, it is far from being fast and therefore cheap.

If there are strong indications of where the passage might be, making profiles perpendicular to the supposed structure is a really good idea.

On the other hand, if nothing can be said, doing the profiles blindly is really risky, as you can easily miss the target. In this case, the fast acquisition availaible with GPR or magnetics (we talk about thousand of square meters per hour with profiles every meter) seems for me to be a better first choice, at least to target the best places to conduct further measurement such as ERT (or else)

You can use GPR if the soil is of low conductivity, using shielded antennas 100, 200 or 300 mhz. It would result in real time unlike the geoelectric tomography.

If the soil is highly conductive, you can use seismic refraction obtained with a minimum of 5 dromo- chronic and make seismic tomography (eg using a 24-channel Geode Equipment)

I agree with Germán. Tomography could be bether if you have space enought to put electrodes in soil, but with all the houses maybe you can´t. I have good results with GPR till 3-4 m deep.

Electric tomography or low frequency GPR - as mentioned above. Try the new shielded 80 MHz antenna from MALA Geoscience or their RTA snake antenna (available for rental in Greece).

Contact Prof. Tsokas from Thessaloniki University, who is the most renowned expert on electrical tomography surveys of burial mounds, currently working on the Amphiopolis mound. 

Check with Dr. Klisthenis Dimitriadis from Geoservice Athens, who recently investigated the Tholos Tomb of Acharnon in the Athens area.

Hi,

If the ground conductivity is low enough GPR is defently the best and the fastest one. Since you are loking in the range of 12 m a 300Mhz and 200Mhz anthennas might be on the limit of range of detection. Therfore 100Mhz is better. You might have detectabilty of 10% of the target depth in target diameter. It means that at 12m you might detect about 1.2m tunnel diameter. There are other issues such as difractions patterns or ressonance effects in cavities (I have investigate these issues some years ago).

You must use in this environment only shielded antennas not other unshielded antennas as mentioned. I would recomand to do a 3D survey but 2d transects to the tunnel route will do the job. If you use 3D I would use GPR Slice.

ERT will not do the job!!! The most common pattern is Wenner (which provide eaqual spacing between the arraeys of the 4 ekectrodes. The penetration is 1/5 of the spread of the 4 specific electrodes. It means that in single point to penetrate to 12 m you will need 60m of seperation between the outer AB electrodes (inject the DC currents) and 20m seperation between the MN electrodes (the potential electrodes). in order to get a 1m+ tunnel diameter it is impossible from physical aspects.

Some one in the forum sent an example of tunnel detection of ~2m diameter in 2 m depth! this is not your case.

If you want to consult with me please send an email to [email protected]

I/we have a very extensive experience in such targets.

Best regards

Amit

We have extensive experience in prospecting artificial hills in the Republic of Altai with geoelectric imaging using Wenner-Schlumberger  array and in Greece at the Amphipolis Hill. It works well.

The tomographic georadar works well as well with 100 and 80 Mhz antennas.

I am grateful to all contributors for their suggestions. I thank everyone for their input and I am looking forward to apply your scientific achievements into uncovering the hidden mysteries at this location

In addition you could use seismic tomography as long as you can vanish the city's sound noise with repetitive shooting at every point along the seismic chosen line. You need a hand held vibrating instrument to bang the ground to help you to get a very large number of seismic profiles per shot point. Then you need to induce stacking procedures in order to extract the clear seismic record free entirely of the city's noisy environment. We have applied this technique in detecting King Xerxes' Canal in East Chalkidik;i peninsula In Greece. Let me know more about the case you investigate.